1. Field of the Invention
The present invention relates to a tantalum solid-state electrolytic capacitor and a fabrication process therefor, more particularly to a tantalum solid-state electrolytic capacitor in which a conductive high polymeric compound is formed through chemical oxidation polymerization as a solid-state electrolyte.
2. Description of the Related Art
In general, a tantalum solid-state electrolytic capacitor is constituted of a sintered body obtained through sintering of tantalum powder, a tantalum oxide layer formed on the surface of the sintered body, a solid-state electrolyte on the tantalum oxide layer, and a conductive layer, such as a graphite layer, a silver layer and so forth, formed on the solid-state electrolyte. The capacitor is polarized, in which a metal tantalum generally becomes an anode side electrode and the conductive layer on the solid-state electrolyte becomes a cathode side electrode.
Here, as the solid-state electrolyte, manganese dioxide has conventionally been frequently used. However, in recent years, for the purpose of improvement of capacitor characteristics and reliability, there has been proposed the solid-state electrolyte formed of a conductive high polymeric compound, such as polypyrrole and so forth in place of the manganese dioxide. Such a proposal has been disclosed in Japanese Examined Patent Publication (Kokoku) No. Heisei 4-56445 (Japanese Patent Application No. Showa 58-144374), for example.
The solid-state electrolytic capacitor employing the conductive high polymeric compound as the solid-state electrolyte has the following superior characteristics in comparison with one employing manganese dioxide, as pointed out in the foregoing publication.
(1) Since the conductivity of the conductive high polymeric compound is several tens times greater than that of the manganese dioxide, it has excellent high frequency characteristics. Therefore, the conductive high polymeric compound is desirable for its capability of adaption for recent rising of operation frequency of electronic devices.
(2) As pointed out in the above-identified publication, since in the capacitor employing the manganese dioxide, the manganese dioxide is produced by pyrolysis of manganese nitrate, thermal stress of 200.degree. to 300.degree. C. is applied to the sintered body (already formed with the surface oxide layer) during production process. Therefore, defects in the surface oxide layer can cause increased leakage current. In contrast to this, the capacitor employing the conductive high polymeric compound as the solid-state electrolyte, does not require such heat treatment at high temperature during production process, Therefore, the surface oxide layer cannot be damaged so as to have lesser variation of characteristics and occurrence of failure in long period use. Therefore, the conductive high polymeric compound may provide higher reliability.
(3) When defects are caused in the surface oxide layer, since the conductive high polymeric compound has lower self-healing temperature than that of the manganese dioxide, the maintenance of the surface oxide layer can be performed in fine leakage current regions. Even at this point, the conductive high polymeric compound may provide higher reliability.
The present invention relates to the tantalum solid-state electrolytic capacitor, in which the conductive high polymeric compound is employed as a solid-state electrolyte.
In the solid-state electrolytic capacitor of this type, there are generally two production processes for producing the conductive high polymeric compound as the solid-state electrolyte, i.e., a method employing electrolytic oxidation polymerization and a method employing chemical oxidation polymerization. In the case of the electrolytic oxidation polymerization, since the surface oxide layer as a dielectric body is electrically insulative, it is difficult to directly form the conductive high polymeric compound on the surface oxide layer. Therefore, it is inherent to form some pre-coat having conductivity, such as manganese dioxide, conductive high polymeric compound layer formed by a chemical oxidation polymerization process or so forth on the surface oxide layer and to form the conductive high polymeric compound with externally contacting an electrode to the conductor thus formed. This makes the production process complicated.
In the chemical oxidation polymerization process as another method for forming the conductive high polymeric compound, there is a process (1) for forming the conductive high polymeric compound by alternately dipping into oxidizing material solution and monomeric solution and a process (2), in which a solution of a mixture of the oxidizing material and monomer is maintained at a temperature lower than a reaction temperature to progress polymerizing reaction, after dipping the sintered body (on which the surface oxide layer is already formed) in the mixture solution, the sintered body is removed from the mixture solution and subject to a temperature higher than or equal to the temperature at which polymerization is initiated to progress polymerization is progressed.
FIG. 1 diagrammatically illustrates sectional construction of the conventional tantalum solid-state electrolytic capacitor employing the conductive high polymeric compound as the solid-state electrolyte. As shown in FIG. 1, a sintered body (tantalum plate 1 in FIG. 1) has a large surface area due to pore 1a formed through sintering of the tantalum powder. On the surface of the tantalum plate 1 (surface of the pore 1a), a thin tantalum oxide layer (Ta.sub.2 O.sub.5) 2 is formed. Furthermore, on the thin tantalum oxide layer 2, the conductive high polymeric compound 3 as solid-state electrolyte is formed. On the conductive high polymeric compound 3, a cathode side electrode 4 is formed. As the cathode side electrode 4, a conductor formed by laminating a graphite layer and a silver layer in order. In relation to the present invention, the conventional capacitor is characterized in that the conductive high polymeric compound 3 completely fills the pore 1a on the surface of the tantalum plate 1.
As set forth above, the conventional tantalum solid-state electrolytic capacitor employing the conductive high polymeric compound as the solid-state electrolyte has been considered to have superior high frequency characteristics and higher reliability in comparison with the capacitor employing the manganese dioxide.
However, through investigation made by the inventors, it has been found that both capacitors, i.e., the tantalum solid-state electrolytic capacitor and the capacitor employing manganese dioxide have substantially equal reliability. For instance, when a loaded life test (a test for reliability with loading a voltage for a long period) was performed as a test for reliability, there was no substantial difference in the rate of occurrence of failure between the capacitors. Also, the mode of failure in the tantalum solid-state electrolytic capacitor was heating due to large leak current, firing of the tantalum plate 1 as the solid-state electrolyte or the external resin cover layer (not shown) due to the heat and therefore, no significant difference to the capacitor employing the manganese dioxide can be observed.